I. Technical Field
The present invention relates to cellular radio communication networks, and particularly to power control for achieving soft handover.
II. Related Art and Other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. Different types of control channels may exist between one of the base stations and user equipment units (UEs), such as (as one example) a common pilot channel (CPICH).
In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which is in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a wideband code division multiple access (W-CDMA) system.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.”
Soft handover thus means that the radio links are added and removed in a way that the user equipment unit always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “Iu” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “Iub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface”. In some instances, a connection involves both a Serving or Source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the connection but with one or more diversity legs of the connection being handling by the DRNC. The interface between a SRNC and a DRNC is termed the “Iur” interface.
In general, a CDMA base station attempts to maintain the same received power level in communications with each mobile station that it currently serves. To this end, a base station measures the received signal from each of the mobile stations in order to determine a parameter known as the Signal to Interference Ratio (SIR). For each mobile station, the SIR is compared with a target value SIR. If the SIR measured for a particular mobile station is less than the target value SIR, the base station commands the mobile station to increase its power in order that a stronger signal can be received at the base station. On the other hand, if the SIR determined for the particular station is greater than the target value SIR, the base station requests the mobile station to decrease its power. Thus, a power control loop (“inner power control loop”) is established between the base station and the mobile station. An uplink aspect of the inner power control loop involves the mobile station transmitting to the base station. Upon receiving the signal from the mobile station, the base station compares the SIR for the station with the target SIR and provides power control commands to the mobile station as a downlink aspect of the power control loop.
The target SIR must be continually updated for numerous reasons, e.g., increase and decrease of the number of mobile stations served by the base station. The updating of the target SIR typically is part of another control loop, in particular a quality control loop or “outer power control loop” between the base station and the radio network controller (RNC). In an uplink aspect of the quality control loop, the base station provides the radio network controller (RNC) with an indication of the quality of connection for each of the mobile stations currently served. The radio network controller (RNC) uses such quality indication in order to calculate or otherwise determine the updated SIR target value. The updated SIR target value is then transmitted to the base station on a downlink aspect of the quality control loop. Examples of uplink power control are provided in U.S. Pat. No. 5,623,484 to Muszynski and U.S. Pat. No. 6,154,450 to Wallentin et al, both of which are incorporated herein by reference.
The UMTS specifications (see, e.g., 3 GPP TS 25.214 version 6.5.0, incorporated herein by reference) discuss criteria and techniques for determining whether or not a connection with a mobile, i.e., user equipment unit (UE), is having reception quality good enough for communication or not. Such determination is typically based on ability to obtain synchronization with the user equipment unit.
Cell breathing and soft handover are two central characteristics and mechanisms in CDMA systems. “Cell breathing” means that the cell coverage is varying with the cell load, and is discussed, e.g., in U.S. patent application Ser. No. 09/385,375, filed Aug. 30, 1999, and entitled “CELL BREATHING REDUCTION FOR TELECOMMUNICATIONS”, which is incorporated by reference herein in its entirety.
In UTRAN, a mobile station is requested to report measurements related to base stations in the ‘monitored set’—a set of base stations that is updated every time the connected set of base stations is updated. For example, the mobile is connected to cell B, and monitors all cells in the monitored set. When cell A becomes a sufficiently good candidate, the mobile station reports such to the radio network controller (RNC) in a measurement report. The radio network controller (RNC) responds by adding cell A to the active set via the RRC message Active Set Update, which is confirmed by the mobile station. Then the RNC updates the ‘monitored set’ in the mobile by the RRC message Measurement Control. The scheme is then repeated as necessary.
“Soft handover” implies that users may gradually leave one cell and enter another cell. Users enter and leave soft handover based on relative measurements of common pilot channel (CPICH) quality for the base stations. In an ideal case with simplistic propagation, the received CPICH RSCP (Received Signal Code Power) degrades as 1/r4, where “r” is the distance to the base station. The CPICH RSCP is equal halfway between the base stations, and also the uplink RSCP is equal in two base stations when the UE is halfway.
The uplinks and downlinks are said to be balanced if the point where the uplink RSCPs are equal coincides with the point where the CPICH RSCPs are equal. In order to match the cell coverage to the available resources, CPICH powers can be tuned to adapt the relative number of users that are allocated to the base stations. However, this adaptive tuning also means that the balance is lost. In addition to different CPICH power settings, the imbalance between uplink and downlink can also result from or depend on such factors as the feeder losses and the use of tower mounted low-noise amplifiers, for example. Balancing of uplink and downlink power is addressed, albeit differently, in Swedish Patent Application 0402003-8, filed Aug. 6, 2004, which is incorporated herein by reference.
FIG. 1A and FIG. 1B are graphs showing (downlink) CPICH RSCP and uplink (UL) RSCP (in watts power), respectively, for two example base stations (a left base station and a right base station) as functions of distance relative to the two base stations. In both FIG. 1A and FIG. 1B, the “left” base station can be conceptualized as located at the leftmost position of the X axis (e.g., X=0) while the “right” base station can be conceptualized as located at the rightmost position of the X axis (e.g., X=1000). The X axis is labeled in distance units from the left base station. In FIG. 1A the curve 1A-L represents the CPICH RSCP as a function of distance from the left base station, while the curve 1A-R represents the CPICH RSCP as a function of distance from the right base station. With regard to the right base station, the CPICH RSCP (FIG. 1A) increases along the X axis. Similarly, in FIG. 1B the curve 1B-L represents the UL RSCP as a function of distance from the left base station, while the curve 1B-R represents the UL RSCP as a function of distance from the right base station.
In FIG. 1A, a soft handover region has a leftmost end depicted by vertical dashed line SOL-A and a rightmost end depicted by vertical dashed line SOR-A. In FIG. 1B, a soft handover region has a leftmost end depicted by vertical dashed line SOL-B and a rightmost end depicted by vertical dashed line SOR-B. The farther a mobile station travels (to the left) from the right base station, the lower the CPICH RSCP becomes for the right base station. Eventually, a measurement report is triggered, which cause the left cell of the left base station to be reported and eventually added to the active set. As the mobile station travels even further to the left, the right cell is dropped from the active set, so that only the left cell remains, which gradually experiences an improving CPICH RSCP.
In the situation shown in FIG. 1A and FIG. 1B, the left cell or left base station has a 3 dB higher CPICH power than the right cell, which means that this left cell will attract more users than the right cell. This also creates an imbalance in the network. This imbalance is exemplified in FIG. 1B wherein uplink RSCP to the left cell is weak at a rightmost end of the soft handover region (e.g., at vertical dashed line SOR-B).
The imbalance illustrated by FIG. 1A and FIG. 1B is critical when moving from right to left, since the leftmost cell will have difficulties to synchronize to the mobile station at these low levels—especially if the CPICH power difference is large. This could for example be the case in a macro-micro deployment.
The uplink RSCP differences are emphasized in FIG. 2A and FIG. 2B, which are similarly constructed and labeled as FIG. 1A and FIG. 1B for a left base station and a right base station. FIG. 2A and FIG. 2B depict RSCP values when moving from one cell to another (moving and soft handover from left to right in FIG. 2A; moving and soft handover from right to left in FIG. 2B). When the mobile station is essentially exactly between the two base stations at X=500, both base stations receive from the mobile station at the same level. The path losses from both base stations are also equal at this location. However, CPICH RSCP is different, since there is a difference in CPICH power (3 dB). This means that based on downlink measurements, the mobile station measures equal CPICH RSCP from both base stations at x=550, i.e. the left cell is experienced as the larger of the two cells.